The Guardian has compiled detailed evidence of alleged war crimes committed by Israel during the 23-day offensive against Gaza earlier this year, involving the use of Palestinian children as human shields, the targeting of medics and hospitals, and drone aircraft firing on civilians.

Three Guardian films based on a month-long investigation, add weight to calls this week for a full inquiry into the events surrounding Operation Cast Lead, which was aimed at Hamas but left about 1,400 Palestinians dead, including up to 300 children.

The Israeli Defence Forces (IDF) refused to respond directly to the allegations made against its troops, but issued statements denying the charges, and insisted international law had been observed.

The Guardian’s investigation follows soldiers’ evidence published in the Israeli press about the killing of Palestinian civilians and complaints by colleagues involved in the military operation that the rules of engagement were too lax.

Amnesty International has said Hamas should be investigated for executing at least two dozen Palestinian men in an apparent bout of score-settling with rivals and alleged collaborators while Operation Cast Lead was under way.

Human rights groups say the vast majority of offences were committed by Israel, and that the Gaza offensive was a disproportionate response to Hamas rocket attacks. Since 2002, there have been 21 Israeli deaths from Hamas rockets fired from Gaza and during Operation Cast Lead there were three Israeli civilian deaths, six Israeli soldiers killed by Palestinian fire and four killed by friendly fire.

"Only an investigation mandated by the UN security council can ensure Israel’s co-operation and it’s the only body that can secure some kind of prosecution," said Amnesty’s Donatella Rovera, who spent two weeks in Gaza investigating war crimes allegations. "Without a proper investigation there is no deterrent. The message remains the same : ’It’s OK to do these things – there won’t be any real consequences.’"

Some of the most dramatic testimony gathered by the Guardian came from three teenage brothers in the al-Attar family. The trio describe how they were taken from their home at gunpoint, made to kneel in front of tanks to deter Hamas fighters from firing at them and sent by Israeli soldiers into Palestinian houses to clear them.

"They would make us go first so if any fighters shot at them the bullets would hit us not them," 14-year-old Al’a al-Attar said.

Medics and ambulance drivers said they were targeted when they tried to tend to the wounded. Sixteen of them were killed. According to the World Health Organisation, more than half of Gaza’s 27 hospitals and 44 clinics were damaged by Israeli bombs. Two clinics were destroyed. In one incident, paramedics were fired on by a tank using a shell filled with 8,000 lethal metal darts as they were carrying a wounded man to an ambulance.

In a report released today, doctors for Human Rights Israel said there was "certainty" that Israel violated international humanitarian law during the three-week war in January, with attacks on medics, damage to medical buildings, indiscriminate attacks on civilians and delays in medical treatment for the injured.

"We have noticed a stark decline in IDF [Israeli Defence Forces] morals concerning the Palestinian population of Gaza, which in reality amounts to a contempt for Palestinian lives," said Dani Filc, chairman of Physicians for Human Rights Israel.

The Guardian gathered testimony of missile attacks by Israeli drones on clearly distinguishable civilian targets. In one case a family of six was killed when a missile hit the courtyard of their house. Israel has not admitted to the use of drones but military experts say their optical equipment is good enough to clearly identify individual items of clothing worn by targets.

The Israeli military issued a statement in response to the allegations saying : "The IDF operated in accordance with the rules of war and did the utmost to minimise harm to civilians uninvolved in combat. The IDF’s use of weapons conforms to international law."

The IDF said an investigation was under way into allegations that hospitals were targeted. A statement said Israeli soldiers were under standing orders to avoid harming medics, but added : "However, in light of the difficult reality of warfare in the Gaza Strip carried out in urban and densely populated areas, medics who operate in the area take the risk upon themselves."

The use of human shields was outlawed by Israel’s supreme court in 2005 after a string of incidents. The IDF said only Hamas used human shields by launching attacks from civilian areas.

An Israeli embassy spokesman said any allegations from Gaza were suspect because of Hamas pressure on witnesses. "Anyone who understands the realities of Gaza will know that these people are not free to speak the truth. Those that wish to speak out cannot for fear of beatings, torture or execution at the hands of Hamas," the spokesman said in a written statement.

However, the accounts gathered by the Guardian are supported by the findings of human rights organisations and soldiers’ testimony published in the Israeli press.

An IDF squad leader is quoted in the daily newspaper Ha’aretz as saying his soldiers interpreted the rules to mean "we should kill everyone there [in the centre of Gaza]. Everyone there is a terrorist.

"To write ’death to the Arabs’ on the walls, to take family pictures and spit on them, just because you can," the squad leader said. "I think this is the main thing : To understand how much the IDF has fallen in the realm of ethics, really. It’s what I’ll remember the most."

Last week, a group of 16 of the world’s leading war crimes investigators and judges called on the UN to launch a full inquiry into "alleged gross violations of the laws of war committed by both sides during the recent conflict in Gaza and southern Israel".